As you can see, the avatars are added by function bp_group_list_admins
in this file:
buddypress\bp-templates\bp-legacy\buddypress\groups\single\group-header.php
Unfortunately, that function has no filters.
So you’ll need to create a template over-ride of group_header.php in your theme.
Then replace bp_group_list_admins() with a custom function that you create in plugins/bp-custom.php
function bp_group_list_admins lives in
\buddypress\bp-groups\bp-groups-template.php
You can copy it and change the function name and the html output of the foreach loop.
You’ll probably need to do the same thing for bp_group_list_mods()
